FIG. 1 illustrates a conventional power supply 110 that is used in induction heating or melting applications. The power supply consists of an ac-to-dc rectifier and filter section 112, a dc-to-ac inverter section 120 and a tuning capacitor section 130. For the power supply shown in FIG. 1, a three-phase diode bridge rectifier 114 converts three-phase (A, B, C) ac utility line power into dc power. Current limiting reactor L108 smoothes out the ripple in the output dc current of the rectifier, and capacitor C108 filters the ac component from the output dc voltage of the rectifier. The filtered dc output of the rectifier is inverted to ac by a full-bridge inverter consisting of solid state switches S101, S102, S103 and S104 and associated antiparallel diodes D101, D102, D103 and D104, respectively. Alternating turn-on/turn-off cycles of switch pairs S101/S103 and S102/S104 produce a synthesized ac inverter output at terminals 3 and 4.
Induction load coil L101, represents the power coil used in the induction heating or melting application. For example, in an induction furnace, load coil L101, is wound around the exterior of a crucible in which metal charge has been placed. In an induction heating application, a metal workpiece, such as a strip or wire, may travel through a helical winding of load coil L101, or otherwise be brought near to the coil to inductively heat the workpiece. Current supplied by the power supply and flowing through load coil L101 creates a magnetic field that either directly heats the metal charge or workpiece by magnetic induction, or heats the workpiece by heat conduction from a susceptor that is heated by magnetic induction. Load coil L101, whether it be a single coil or an assembly of interconnected coil sections, has a very low operating power factor. Because of this, a tuning capacitor (or bank of capacitors), such as capacitor C101 must be provided in the load coil circuit to improve the overall power factor of the load coil circuit. These tuning capacitors are a significant cost and volume component of the power supply. Therefore, there exists the need for a power supply for inductive heating or melting applications that utilizes smaller and less costly tuning capacitors.
An objective of the present invention is to provide a power supply for inductive heating or melting applications that utilizes a capacitor connected between the output of the rectifier and the input of the inverter to form a resonantly tuned circuit with the induction load coil used in the application.